Studies have been initiated to identify phosphorylation sites in plant enzymes using peptide mass mapping and MALDI-MS analysis as one experimental approach. Enzymes of immediate interest are sucrose-phosphate synthase (SPS) and NADHnitrate reductase (NR). Both enzymes are inactivated by seryl phosphorylation and peptide mapping analysis has suggested that multiple phosphorylation sites may be involved. These enzymes play key regulatory roles in pathways involved with carbohydrate and nitrogen metabolism, respectively, and phosphorylation may be a critical mechanism controlling some aspects of C/N balance. Consequently, it is important that phosphorylation sites be identified.